1. Field of the Invention
The present invention relates to an improvement of an optical sheet used for controlling an illumination light path mainly in a backlight unit for a display which uses a liquid crystal display element, and in particular, to a backlight unit and a display which are equipped with the optical sheet.
2. Description of the Related Art
As displays which are typified by liquid crystal displays, the popularization of a type in which a light source required for recognizing presented information is built in has been remarkable. In a battery-powered device such as a laptop computer, the electric power consumed by the light source accounts for a considerable percentage of the electric power consumed in the entire battery-powered device.
Accordingly, the battery life is increased by reducing the total electric power required for providing a predetermined luminance. This is particularly favorable for a battery-powered device.
The Brightness Enhancement Film (BEF) which is a registered trademark of a U.S. company, 3M, has been broadly used as an optical sheet for solving this problem.
As shown in FIG. 1, the BEF is a film in which unit prisms 72 having a triangular-shaped cross section are arrayed periodically in one direction on a single component 70.
The unit prisms 72 have sizes (a pitch) which are larger than a wavelength of light.
The BEF condenses “off-axis” light and redirects it “on-axis” toward a viewer or recycles the light.
At the time of using the display (turn on), the BEF increases the on-axis luminance by reducing the off-axis luminance. The term “on-axis” here is a direction coincident with a visual direction of a viewer, and is generally along a normal line direction with respect to a display screen (the direction F shown in FIG. 1).
When the repeating array structure of the unit prisms 72 is a parallel structure in only one direction, it is possible to only redirect or recycle light in the parallel direction, and in order to carry out luminance control of a display light in horizontal and vertical directions, two sheets are used in combination so as to be superimposed on one another such that the parallel directions of the prism groups are substantially perpendicular to one another.
In accordance with such use of the BEF, it is possible for a display designer to achieve a desired on-axis luminance while reducing an electricity consumption.
As Patent Documents which disclose a technology of using a luminance control component having a repeating array structure of the unit prisms 72 typified by the BEF, for a display, there have been a number of Patent Documents, as shown by, for example, Jpn. Pat. Appln. KOKOKU Publication No. 1-37801, Jpn. Pat. Appln. KOKAI Publication Nos. 6-102506 and 10-506500.
In an optical sheet using the BEF as a luminance control component as described above, as shown in FIG. 2, it is possible to control so as to enhance a light intensity in the visual direction F of a viewer by emitting a light beam L from a light source 20 finally at a controlled angle φ by refraction x. However, at the same time, optic elements due to reflection/refraction Y are emitted wastefully in a cross direction without heading in the visual direction F of a viewer.
Accordingly, in a light intensity distribution emitted from the optical sheet using the BEF as shown in FIG. 1, as shown in the light intensity distribution B of FIG. 3, the light intensity in the visual direction F of the viewer, i.e., at an angle 0° with the visual direction F is enhanced to the maximum. However, as shown by small light intensity peaks in the vicinity of ±90° shown on the abscissa in the drawing, there is the problem that light emitted wastefully from cross directions is increased.
Further, when the above-described component 70 is used for a direct-type system backlight unit in which an optical waveguide structuring an edge-light type surface light source is not used, a structure in which a light diffusion layer (a single light diffusion plate or a combination with a light diffusion film) is placed between the light source 20 and the component 70 is commonly used. However, due that their surfaces are flat, when both components are in contact (the component 70 and the light diffusion layer (not shown)) such that there is not a definite boundary surface and if both are materials having a refractive index close to one another, the light incident on the unit prisms 72 is not incident at the intended angle, which makes it difficult to perform the optical characteristic as designed by the unit prisms 72.
In particular, in the case of using a structure in which the component 70 and the light diffusion layer are laminated to be integrated via an adhesive layer or an adhesion layer, generation of unexpected elements of incident light (or a reduction in functions due to the light diffusion layer) is easily brought about, which makes it further difficult to perform the optical characteristic as designed.
The present invention has been achieved in consideration of the circumstances, and an object of the present invention is to provide an optical sheet in which it is possible to emit light from a light source so as to be uniformed, and to control a diffusion range without increasing an amount thereof emitted wastefully.
Note that, in the present application, “diffusion” and “scattering” serving as an optical characteristic, “adhesion layer” and “adhesive layer”, and “optical sheet” and “optical film” according to the present application are used as synonyms.